The present invention relates to apparatuses for controlling the air-fuel ratio of air-fuel mixtures used during combustion in engines. More particularly, the present invention pertains to learning apparatuses and methods for optimally controlling the air-fuel ratio in engines incorporating purge apparatuses, which burn the fuel vapor in fuel tanks during combustion so that the fuel vapor is prevented from being released into the atmosphere.
Three-way catalysts, which convert engine emissions into harmless emissions, are widely used in automobile engines, the emissions of which are required to be highly purified. A three-way catalyst oxidizes carbon monoxide (CO) and hydrocarbon (HC) and reduces nitrogen oxide (NO.sub.x). The threeway catalyst converts carbon monoxide to carbon dioxide (CO.sub.2), hydrocarbon to water (H.sub.2 O) and carbon dioxide (CO.sub.2), and nitrogen oxide to oxygen (O.sub.2) and nitrogen (N.sub.2). For the three-way catalyst to function effectively, the air-fuel ratio of the air-fuel mixture burned in the engine must be in the proximity of the stochiometric air-fuel ratio. That is, the air-fuel ratio must be in an extremely narrow range. Therefore, prior art three-way catalysts require the air-fuel ratio to be controlled with high precision so that the ratio remains stochiometric. Accordingly, the basic fuel injection amount corresponding to the stochiometric air-fuel ratio in each engine operating state (e.g., engine speed and intake air amount) is stored in the form of a map. The air-fuel ratio obtained from the map, or the basic air-fuel ratio, and the stochiometric air-fuel ratio are theoretically equal to each other. However, wear and dimensional tolerances of components related with the air-fuel ratio control, such as airflow meters or injectors, may cause the basic air-fuel ratio to deviate from the stochiometric, or target, air-fuel ratio. Thus, a learning process is carried out to correct such deviation when controlling the air-fuel ratio.
Recent engines employ purge apparatuses to collect the fuel vapor produced in fuel tanks and to prevent the fuel vapor from being released into the atmosphere. The collected fuel vapor is sent to the engine for combustion, or purged.
When controlling the air-fuel ratio in an engine provided with a purge apparatus, the purged volume of the fuel vapor must be taken into consideration.
Air-fuel ratio control that reflects the influence of the purged fuel vapor is generally executed in the following manner. The basic fuel injection amount corresponding to the operating state of the engine (engine speed and intake air amount) is obtained by referring to a map. The fuel injection amount is then adjusted through feedback control so that the stoichiometric air-fuel ratio is obtained. If the basic fuel injection amount and the actual fuel injection amount differ from each other, a correction co-efficient for correcting the fuel injection amount, or an air-fuel ratio correction coefficient, is stored as a learned value. The learning of the air-fuel ratio correction coefficient takes place when the fuel vapor is not being purged, or during purge-off, so that the air-fuel ratio correction coefficient is not affected by the purged fuel vapor.
The fuel injection amount obtained in correspondence with the target air-fuel ratio when fuel vapor is not being purged differs from the fuel injection amount obtained in correspondence with the target air-fuel ratio during purging. The fuel injection amount difference and the purged amount of fuel vapor in the intake air (i.e., purged rate) are used to compute the concentration of fuel in the fuel vapor, or vapor concentration coefficient, which is stored as a learned value. The product of the purged rate and the vapor concentration coefficient results in a correction coefficient (purge correction coefficient), which reflects the influence of the fuel vapor on the air-fuel ratio. The purge correction coefficient is used to correct the air-fuel ratio. In this manner, air-fuel ratio control is performed by taking into consideration the influence of the fuel vapor.
The frequency of learning the air-fuel ratio correction coefficient must be increased to improve the precision of the air-fuel control. However, the purging of the fuel vapor must be stopped to renew the air-fuel ratio learned correction coefficient. This increases the time during which purging cannot be performed, which may result in insufficient fuel vapor purging. If fuel vapor purging is performed continuously over a long period of time, the number of opportunities for learning the air-fuel ratio correction coefficient decreases. This lowers the accuracy of the learned air-fuel ratio correction coefficient, which lowers the accuracy of the air-fuel ratio control.
Accordingly, for example, Japanese Unexamined Patent Publication No. 7-166978 proposes an air-fuel control apparatus that learns the air-fuel ratio correction coefficient when the fuel concentration of the purged fuel vapor is low. This increases the frequency of learning and therefore increases the accuracy of the air-fuel ratio control.
However, the air-fuel ratio control apparatus proposed in the Japanese patent publication renews the air-fuel ratio correction coefficient assuming that the concentration of the purged fuel vapor is constant. Therefore, if the concentration of the purged fuel vapor changes when the learning process is carried out, the vapor concentration learned before the concentration change is used when correcting the air-fuel ratio. Hence, the concentration change is not reflected in the learning process. As a result, the air-fuel ratio is controlled in accordance with an erroneously learned air-fuel ratio correction coefficient. This decreases the accuracy of the air-fuel ratio control.